1. Field of the Invention
The invention relates to a process for the production of mouldings from a mixture of particulate natural materials and thermoplastic binder, the mixture being shaped into a moulding by means of increased temperature and/or pressure.
2. Description of the Related Art
Natural fibres, in particular wood fibres, have recently increasingly been used in combination with thermoplastics for the production of wood plastic composites. For this purpose, the wood fibres or wood flour are mixed with a thermoplastic binder and processed by means of extrusion to give mouldings. WO-A 96/34045 discloses a process for the production of mouldings, in which natural fibre materials are processed as a mixture with thermoplastics. The preparation of plastic moulding materials by extrusion of a mixture of natural fibre and thermoplastic is also disclosed in WO-A 03/008494. The natural fibre content is introduced in the form of pellets bound with thermoplastic.
These natural substances which are present, for example, in the form of wood fibres, wood flour or woodchips, contain a considerable amount of residual moisture in the air-dry state. Thermoplastics, such as polyolefins, polyvinyl chloride or ABS, are not capable of absorbing the water released during processing in an amount significant for the process. In thermoplastic processing of mixtures of such natural substances and thermoplastics, for example by means of extrusion, the residual water present therefore leads to uncontrolled bubble formation in the mouldings, owing to the abrupt vaporization occurring at the processing temperatures. This can take place to such an extent that the cohesion of such a moulding is no longer guaranteed.
WO-A 03/035373 and WO-A 2004/090022 therefore propose additionally using a water-binding biopolymer, starch or protein, in the production of mouldings from natural fibre and thermoplastic. A disadvantage thereby is that the mixture also comprises a third component in addition to the natural fibre and the thermoplastic binder, and the strength of the moulding is reduced.